The Lewis diagram for BH2- is: The electron-pair geometry around the B atom in BH2- is fill in the blank 4. There are lone pair(s) around the central atom, so the geometry of BH2- is fill in the blank 6.
The Correct Answer and Explanation is:
Correct Answers:
- The electron-pair geometry around the B atom in BH₂⁻ is trigonal planar.
- There is one lone pair around the central atom, so the geometry of BH₂⁻ is bent.
Explanation (300 words, textbook-style):
The molecular structure of the BH₂⁻ ion can be understood through its Lewis diagram and application of the Valence Shell Electron Pair Repulsion (VSEPR) theory. The central atom is boron (B), which normally has three valence electrons. Each hydrogen atom contributes one electron through a single covalent bond. The additional negative charge on the ion represents an extra electron, bringing the total number of valence electrons in the molecule to eight.
In drawing the Lewis structure, the two hydrogen atoms are bonded to the boron atom via single bonds. After bonding, four electrons remain, which are placed as two lone pairs. However, boron tends to defy the octet rule due to its small size and incomplete octet preference. In this case, one lone pair is placed on boron to maintain stability while the remaining electrons are distributed accordingly to satisfy the structure’s charge and bonding pattern.
According to VSEPR theory, the electron-pair geometry considers both bonding and lone pairs around the central atom. With two bonding pairs (from hydrogen) and one lone pair on boron, the total number of regions of electron density is three. Three regions of electron density arrange themselves in a trigonal planar geometry to minimize repulsion.
However, molecular geometry refers only to the positions of atoms, not lone pairs. The presence of one lone pair and two bonding pairs gives the ion a bent molecular shape, similar to that seen in molecules like SO₂. The lone pair occupies more space than bonding pairs, leading to a bond angle slightly less than 120°.
Thus, the electron-pair geometry is trigonal planar, and the molecular geometry of BH₂⁻ is bent, with one lone pair on the central atom.
